A variety of electrochemical devices require processes and compositions for providing seals on or within the devices. The seals may be used to encapsulate the entire device, or they may separate various chambers within the device. As an example, many types of seal materials have been considered for use in high-temperature rechargeable batteries/cells for joining different components.
Sodium/sulfur or sodium/metal halide are good examples of high-temperature batteries that may include a variety of ceramic and metal components. The ceramic components often include an electrically insulating alpha-alumina collar, and an ion-conductive electrolyte beta-alumina tube, and are generally joined or bonded via a sealing glass. The metal components usually include a metallic casing, current collector components, and other metallic components which are often joined by welding or thermal compression bonding (TCB). While mechanisms for sealing these components are currently available, their use can sometimes present some difficulty. For example, metal-to-ceramic bonding can be challenging, due to thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic and metal components.
Since metal-to ceramic bonding is most critical for the reliability and safety of the cells for high-temperature batteries, many different types of seal materials and sealing processes have been considered for joining such components, including ceramic adhesives, brazing, and sintering. However, most of the seals may not withstand high temperatures and corrosive environments. A common bonding technique involves multiple steps of metalizing the ceramic component, followed by bonding the metallized ceramic component to the metal component using thermal compression bonding (TCB).
The bond strength of such metal-to-ceramic joints is controlled by a wide range of variables, such as the microstructure of the ceramic component, the metallization of the ceramic component, and various TCB process parameters. In order to ensure good bond strength, the process requires close control of several parameters involved in various process steps. In short, the method is relatively expensive, and complicated, in view of the multiple processing steps, and the difficulty in controlling the processing steps.
In some instances, metallization of a ceramic surface for bonding with a metal component involves the use of molybdenum or molybdenum/manganese-based inks, as described in “Zebra Electric Energy Storage system: From R&D to Market”, Renato Manzoni et al, HTE hi.tech.expo—Milan 25-28 Nov. 2008 (website record). In a typical process, the ink or paste is formulated with various alcohols, amines, and alkanes, along with binder materials. (Various other components may also be present, e.g., ether esters, aromatic compounds, and functionalized silanes). The inks are often screen-printed on top of the ceramic, effectively metallizing the component. The ceramic can then be clamped to an appropriate metal component, e.g., a nickel ring for a battery cell, followed by heating in a kiln or other suitable furnace to join the parts by a TCB technique.
While the use of the molybdenum inks can be useful in some types of metallization applications, they can also present various disadvantages. For example, some of the binder materials present in the inks may remain in the composition, even after high-temperature brazing. The binder residue can hinder the brazing process, and decrease the strength of the overall braze joint. In the case of assembling sodium nickel halide cells, the residue can also adversely affect the reaction chemistry for the cells, if contact is made with the reaction components. Moreover, the ink processes will still usually be part of a TCB process, with its attendant disadvantages, as noted previously.
With these considerations in mind, new types of sealing structures and compositions for energy storage devices and other types of electrochemical cells would be welcome in the art. The new technology should provide hermetic sealing with a joint strength sufficient to meet rigorous end use requirements for the cell. Moreover, the overall sealing structure should be compatible with electrochemical cell contents that might come into contact with the seals. It would also be desirable if the sealing structures can be obtained with relatively low fabrication costs, e.g., as compared to some of the metallization/TCB processes used in conventional situations.